In simplest form, a refrigeration system includes a body of refrigerant material whose state can be changed from gas to liquid and back to gas at practically achievable pressure and at temperatures near a desired temperature. The basic system includes a compressor in which the gaseous refrigerant is compressed and heated. It also includes a heat exchanger, called a condenser, in which the heat of compression is removed from the compressed gas, usually to atmosphere, and where the refrigerant becomes liquid. The condenser is followed in the refrigerant flow path by an expansion apparatus in which the compressed liquid is released into an apparatus where it is subjected to much lower pressure. The liquid can change state and expand, if heated. That circumstance is utilized by adding a second heat exchanger in the system. Called an "evaporator," the second heat exchanger is connected to receive refrigerant from the expansion apparatus. The refrigerant is returned from the evaporator to the compressor to complete the refrigeration circuit and the process cycle. In the process, heat is removed from the environment in the region of the evaporator and transferred to the environment in the region of the condenser.
In an extension of the basic system the process is used to produce and store "cold" whereby the production of cold can be separated in time from use of the cold.
There are several applications in which the time separation of cold production and cold utilization is combined with physical separation of the evaporator from the other elements of the system. The evaporator is combined structurally with the apparatus in which the cold is stored to form a cold storage unit, and the storage unit is made separable from the remainder of the system. An example is found in refrigerated trailer trucks where the compressor and condenser and expansion element are mounted on the tractor and the evaporator and cold storage structure is built into the trailer.
In another example, a cold storage unit is built into an insulated container. The unit and the insulated container are removably connected to the air conditioning system of a passenger car or recreational vehicle.
These examples serve to illustrate the feasibility of refrigeration systems in which cold produced in one place can be stored in a separable cold storage unit for use at a different time and place.